1. Field of the Invention
The invention relates to instruments for measuring magnetic properties of materials, and more particularly, to pickup coils for magnetic hysteresis loop tracers.
2. Description of the Related Art
A magnetic hysteresis loop tracer (for example the SHB Instruments Model 109) operates by applying a strong AC magnetizing field (xe2x80x9cHxe2x80x9d) to a sample under test, and then using a pickup coil surrounding the sample to monitor the response of the sample to the applied field. The response data is used to calculate a hysteresis loop (also known as a B-H curve) that shows the magnetic flux (B) in the sample as a function of the magnetizing field. The properties of the hysteresis loop are displayed by the instrument. The magnetizing field is created by current flowing in a drive coil (or coils). The pickup coil is part of a pickup assembly into which the sample under test is inserted.
One of the fundamental problems involved in the design and implementation of hysteresis loop tracers is the fact that the instrument must ignore the strong applied magnetizing field while detecting, integrating, and displaying the (typically much weaker) field caused by the interaction of the applied field with the sample under test. This is typically accomplished by the use of a single balance coil, positioned some distance xe2x80x9cdownstreamxe2x80x9d from the sample (that is, some distance along an axial centerline of a drive coil or coils) so that it responds to the magnetizing field of the drive coils, but not to the magnetization field from the sample. The signal from the balance coil is subtracted from the signal picked up by the pickup coil that surrounds the sample under test. This provides only a first-order correction, as the balance coil is neither identical to the pickup coil, nor is it positioned in exactly the same part of the magnetizing field as the pickup coil. Additional electronics are used to fine-tune this nulling (or xe2x80x9cbalancingxe2x80x9d) process so that, when properly adjusted, only the magnetization field from the sample is seen and measured by the instrument. Thermally and mechanically induced variations in the pickup and drive coil assemblies can easily interfere with proper balance of the system, and introduce significant measurement errors. This lack of proper balance is often evidenced by a tilt from the horizontal of the displayed hysteresis loop.
The present invention solves these and other problems by providing a geometry for a pickup coil assembly that improves the balance of a hysteresis loop tracer. In one embodiment, first and second balance coils are placed on either side (e.g., top/bottom, left/right, front/back, etc.) of a sample-sensing pickup coil. In one embodiment, the first and second balance coils are wired in series to form a symmetric balance coil that senses the magnetic H field on either side of the pickup coil. In one embodiment, the voltage produced by the symmetric balance coil is subtracted from the voltage produced by the pickup coil.
In one embodiment, the balance coil and the pickup coil are positioned symmetrically with respect to a centerline of a drive coil (or drive coils) to minimize measurement errors due to radial disturbances in the H field produced by the drive coils. Radial symmetry is provided by placing the balance coil on one side of the centerline and by placing the pickup coil on the other side of the centerline adjacent to the balance coil.
In one version, coil forms are used for winding the balance coil(s) and second pickup coil. In one embodiment, the coil forms are physically connected to each other in order to improve thermal and mechanical stability. In one version, the coil forms are constructed from alumina.